Management of genetic diversity in Sahiwal cattle breed in Kenya

Abstract:

The Sahiwal population in Kenya, which is bred under a closed nucleus, is faced with declining effective population size over the years and rate of inbreeding per generation >1% beyond which it should not exceed for a population to maintain its long-term fitness and viability. This study estimated gene origin statistics, Wright’s F-statistics, current and future rates of inbreeding, coancestry and effective population size and genetic gain in lactation milk yield at predetermined rates of inbreeding. The aim was to develop a strategy to manage genetic variability at a predetermined level while achieving desirable genetic progress in the breeding goal. Since the founding population, ancestors contributed equally as shown by the ratio of fe/fa of 0.94, which could also be due to lack of effective selection in this population. The ratio of fg/fa of 0.63 indicated genetic loss of genetic variability occurred through genetic drift in the population. Predicted rate of inbreeding per generation for individuals with at least 3 and 6 complete generations were 0.82 and 1.19% and Ne of 54 and 35, respectively. Parameters of genetic differentiation (Wright’s F-statistics) were FIS=- 0.0071, FST= 0.0036 and FIT =- 0.0034. Loss in genetic diversity for the whole population was -0.023, an indication of random genetic drift and unequal contribution of founders. When the best 2 sires were mated to the best 210 females disregarding average relationship among them (1, 0), genetic gain of 210 kg was realized accompanied by an average inbreeding and rate of inbreeding per generation of 14.5% and 4%, respectively. Restricting average relationship alone (0, -1), resulted in future rate of inbreeding of 1.6% and average merit of 154 kg. Other restrictions of merit and relationship resulted in intermediate values of rate of inbreeding per generation and genetic gain. A rate of inbreeding per generation of <1% was achieved at the same restriction level (0, -1) but using 8 top sires, which resulted in a rate of inbreeding per generation of 0.9 % accompanied by an average merit of 128.2 kg. To achieve a rate of inbreeding per generation of <1%, the size of the breeding population should be increased to at least 8 bulls and 840 females. Practical implications include improving pedigree recording, opening up the nucleus to include other institutional and private herds, in a dispersed open nucleus scheme and appreciation of the depressed genetic gain in the breeding objective.